Every gym has at least one extremely flexible gymnast. Most are born with some “unusual” range of motion and (surprisingly) bending the body through these positions seems to cause no long term damage.
gymnast at Mountain Shadows Gymnastics Camp, Canada
And every circus includes a number of contortionists.
There are a couple of myths acrobatic coaches should dispel:
“Double-jointed” is just a slang expression used to describe the appearance of a person who can bend much further than one might think a joint would allow a limb to bend. In spite of the origin of the word, it is a perfectly acceptable expression to describe a person who is hypermobile.
Since some loose-jointed people are able to pop a joint out of its socket without pain, it may be hard to tell whether a joint is actually dislocated without an x-ray. However, as long as the joint socket is the right shape, most extreme bends can be achieved without dislocating the joint. Actual dislocations are rarely used during athletic contortion acts since they make the joint more unstable and prone to injury, and a dislocated limb cannot lift itself or support any weight.